1. Field of the Invention
The present invention relates to a substrate for a transflective liquid crystal display that is used as a display of a portable electronic apparatus and that is capable of display in both of reflective and transmissive modes and to a liquid crystal display having the same.
2. Description of the Related Art
Liquid crystal displays are generally categorized into transmissive types in which a transparent electrode constituted of an ITO (indium tin oxide) is formed at each pixel and which have a backlight unit on a backside thereof and reflective types in which a reflective electrode made of aluminum (Al) is formed at each pixel. Among recent active matrix liquid crystal displays, reflective liquid crystal displays are drawing attention for their lighter weights, low profiles, and low power consumption. Single polarizer type reflective liquid crystal displays utilizing the TN (Twisted Nematic) mode as disclosed in Japanese Patent Laid-open No. 232465/1993 and Japanese Patent Laid-Open No. 338993/1996 have already been put in use. However, the visibility of a reflective liquid crystal display is greatly dependent on the brightness of the ambience, and a problem arises in that visibility is significantly reduced in a dark place where ambient brightness is relatively low.
A transmissive liquid crystal display exhibits a high contrast ratio and high visibility even in a dark place because it is illuminated from the backside thereof with a backlight unit. However, it has a problem in that visibility is significantly reduced in a place where ambient brightness is relatively high such as the outdoor in good weather (a bright place). Further, since a backlight unit is always used, another problem arises in that power consumption is great.
Liquid crystal displays that solve the above-described problems include front light type reflective liquid crystal displays having a front light unit that provides illumination from the side of the display screen thereof. However, a front light type reflective liquid crystal display exhibits a contrast ratio lower than that of a transmissive liquid crystal display in a dark place because illumination light from the front light unit is reflected by not only the reflective electrodes but also the surface of the display screen. In a bright place, it presents darker display in a bright place compared to a normal reflective liquid crystal display because of light absorption at a light guide plate of the front light unit.
Another approach involves a transflective liquid crystal display in which transflective reflecting films are used as pixel electrodes as disclosed in Japanese Patent Laid-Open No. 333598/1995. In general, metal thin films such as aluminum having a thickness of about 30 nm are used as the transflective reflecting films. However, this results in a reduction in utilization of light because the metal thin films have a high absorption constant. Further, since it is difficult to form transflective reflecting films having a uniform thickness in the plane of a substrate, there will be variations of light transmittance and reflectance in the plane of the substrate.
A transflective liquid crystal display that solves the above-described problems was disclosed in Japanese Patent Laid-Open No. 281972/1999. FIG. 29 shows a configuration of a transflective liquid crystal display according to the related art. As shown in FIG. 29, a plurality of gate bus lines 104 extending in the vertical direction in the figure are formed in parallel with each other on a TFT substrate 102. A plurality of drain bus lines 106 extending in the horizontal direction in the figure are formed in parallel with each other such that they intersect with the gate bus lines 104 with an insulation film which is not shown interposed therebetween. TFTs 108 are formed in the vicinity of the positions where the bus lines 104 and 106 intersect with each other. Drain electrodes 140 of the TFTs 108 are electrically connected to the drain bus lines 106. Source electrodes 142 are electrically connected to reflective electrodes 110 made of aluminum through contact holes 144. The regions where the reflective electrodes 110 are formed serve as reflective regions of respective pixels. Openings are provided in the middle of the reflective electrodes 110 to form transparent electrodes 112 made of ITO. The regions where the transparent electrodes 112 are formed serve as transmissive regions of respective pixels.
FIG. 30 is a sectional view of the liquid crystal display taken along the line X-X in FIG. 29. As shown in FIG. 30, the liquid crystal display is constituted of the TFT substrate 102, an opposite substrate 114, and a liquid crystal layer 116 provided between the substrates 102 and 114. The TFT substrate 102 has a planarization film 120 in reflective regions on a glass substrate 118. A plurality of recesses and projections are formed on a surface of the planarization film 120. Reflective electrodes 110 are formed on the planarization film 120. On a surface of the reflective electrodes 110, there are formed recesses and projections which are associated with the recesses and projections formed on the surface of the planarization film 120 located under the same. The reflective electrodes 110 have improved light scattering characteristics thanks to the plurality of recesses and projections on the surface thereof, and they reflect and scatter external light incident thereupon in various directions.
Transparent electrodes 112 are formed in transmissive regions on the glass substrate 118. The transparent electrodes 112 transmit light emitted by a backlight unit (not shown) provided under the same in the figure. The transparent electrodes 112 are electrically connected to the reflective electrodes 110 through barrier metal layers 136 made of titanium (Ti) or molybdenum (Mo).
The counter substrate 114 has a common electrode 130 that extends throughout a top surface of the glass substrate 119. Polarizers 132 and 134 are applied to surfaces of the substrates 102 and 114 counter to surfaces thereof facing each other, respectively.
The liquid crystal display shown in FIGS. 29 and 30 achieves display in both of the reflective and transmissive modes by forming a reflective region and a transmissive region at each pixel.
In the above-described configuration, however, it is necessary to form both of the reflective electrodes 110 made of Al and the transparent electrodes 112 made of ITO. Further, since corrosion attributable to a battery effect occurs when Al and ITO are formed in contact with each other, the barrier metal layers 136 must be formed between the reflective electrodes 110 and the transparent electrodes 112. This has resulted in a problem in that the liquid crystal display involves complicated manufacturing steps and in that an increase in manufacturing cost occurs.
In the above-described configuration, a reflective region and a transmissive region are formed at each pixel. Therefore, the display exhibits reflection characteristics lower than those of a reflective liquid crystal display and transmission characteristics lower than those of a transmissive liquid crystal display. However, when the area of the reflective regions is increased to achieve improved reflection characteristics, the area of the transmissive regions further decreases to degrade the transmission characteristics further. Similarly, when the area of the transmissive regions is increased to achieve improved transmission characteristics, the area of the reflective regions decreases to degrade the reflection characteristics further. Thus, in a transflective liquid crystal display in the related art, reflection characteristics and transmission characteristics are in the relationship of tradeoff, and a problem has arisen in that it is difficult to improve both of the reflection characteristics and the transmission characteristics.
Further, while light incident upon the reflective regions pass through a color filter (CF) layer twice, the light passes through the CF layer only once in the transmissive regions. This results in a chromatic deviation between display in the reflective mode and display in the transmissive mode. While a chromatic deviation can be optically compensated to some degree, it can degrade display characteristics.